706 ON FORM AND MECHANICAL EFFICIENCY [ch. 



were so conspicuous in all heavy-headed quadrupeds, have now 

 completely vanished. The creature has become bipedal, and body 

 and tail form the extremities of a single balanced cantilever, 

 whose maximal bending-moments are marked by strong, high 

 lumbar and sacral vertebrae, and by iliac bones of peculiar form 

 and exceptional strength. 



Precisely the same condition is illustrated in the Iguanodon, 

 and better still by reason of the great bulk of the creature, and of 

 the heavy load which falls to be supported by the great cantilever 

 and by the hind-legs which form its piers. The long and heavy 

 body and neck require a balance-weight (as in the kangaroo) in 

 the form of a long heavy tail. And the double-armed cantilever, 

 so constituted, shews a beautiful parabolic curvature in the graded 

 heights of the whole series of vertebral spines, which rise to a 

 maximum over the haunches and die away slowly towards the 

 neck and the tip of the tail. 



(5) In the case of some of the great American fossil reptiles, 

 such as Diplodocus, it has always been a more or less disputed 

 question whether or not they assumed, like Iguanodon, an erect, 

 bipedal attitude. In all of these we see an elongated pelvis, and, 

 in still more marked degree, we see elevated spinous processes of 

 the vertebrae over the hind-limbs ; in all of them we have a long 

 heavy tail, and in most of them we have a marked reduction in 

 size and weight both of the fore-limb and of the head itself. The 

 great size of these animals is not of itself a proof against the erect 

 attitude; because it might well have been accompanied by an 

 aquatic or partially submerged habitat, and the crushing stress of 

 the creature's huge bulk proportionately relieved. But we must 

 consider each such case in the whole light of its own evidence; 

 and it is easy to see that, just as the quadrupedal mammal may 

 carry the greater part but not all of its weight upon its fore-limbs, 

 so a heavy-tailed reptile may carry the greater part upon its hind- 

 limbs, without this process going so far as to relieve its fore-limbs 

 of all weight whatsoever. This would seem to be the case in such 

 a form as Diplodocus, and also in Stegosaurus, whose restoration 

 by Marsh is doubtless substantially correct*. The fore-limbs, 



* This pose of Diplodocus, and of other Sauropodous reptiles, has been much 

 discussed. Cf. (int. al.) Abel, 0., Abh. k. k. zool. hot. Ges. Wien, v. 1909-10 (60 pp.) ; 



